1. Field of the Invention
This invention relates to amorphous metal magnetic components, and more particularly, to a high efficiency electric motor having a generally polyhedrally shaped, low core loss, bulk amorphous metal magnetic component.
2. Description of the Prior Art
An electric motor typically contains magnetic components made from a plurality of stacked laminations of non-oriented electrical steel. In variable reluctance motors and eddy current motors, the stators are made from stacked laminations. Both the stator and the rotor are made from stacked laminations in squirrel cage motors, reluctance synchronous motors and switched reluctance motors. Each lamination is typically formed by stamping, punching or cutting the mechanically soft, non-oriented electrical steel into the desired shape. The formed laminations are then stacked and bound to form rotors or stators which have the desired geometry, along with sufficient mechanical integrity to maintain their configuration during production and operation of the motor.
The stator and the rotor in a machine are separated by small gaps that are either: (i) radial, i.e., generally perpendicular the axis of rotation of the rotor, or (ii) axial, i.e., generally parallel to the rotation axis and separated by some distance. In an electromagnetic machine, lines of magnetic flux link the rotor and stator by traversing the gaps. Electromagnetic machines thus may be broadly classified as radial or axial flux designs, respectively. The corresponding terms radial and axial gap designs are also used in the motor art. Radial flux machines are by far most common. The aforesaid punching and stacking methods are widely used for constructing rotors and stators for radial flux motors.
Although amorphous metals offer superior magnetic performance when compared to non-oriented electrical steels, they have long been considered unsuitable for use in bulk magnetic components such as the rotors and stators of electric motors due to certain physical properties and the ensuing impediments to fabrication. For example, amorphous metals are thinner and harder than non-oriented steel, and consequently cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating bulk amorphous metal magnetic components using such conventional techniques, such as punching and stamping, commercially impractical. The thinness of amorphous metals also translates into an increased number of laminations in the assembled components, further increasing the total cost of an amorphous metal rotor or stator magnet assembly.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material, making it very difficult to cut or form easily. Once annealed to achieve peak magnetic properties, amorphous metal ribbon becomes very brittle. This makes it difficult and expensive to use conventional approaches to construct a bulk amorphous metal magnetic component. The brittleness of amorphous metal ribbon may also cause concern for the durability of the bulk magnetic component in an application such as an electric motor.
Magnetic stators are subject to extremely high magnetic forces, which vary rapidly at the frequencies needed for high rotational speed. These magnetic forces are capable of placing considerable stresses on the stator material, and may damage an amorphous metal magnetic stator. Rotors are further subjected to mechanical forces due both to normal rotation and to rotational acceleration when the machine is energized or de-energized and when the loading changes, perhaps abruptly.
A limited number of non-conventional approaches have been proposed for constructing amorphous metal components. For example, U.S. Pat. No. 4,197,146 to Frischmann discloses a stator fabricated from molded and compacted amorphous metal flake. Although this method permits formation of complex stator shapes, the is structure contains numerous air gaps between the discrete flake particles of amorphous metal. Such a structure greatly increases the reluctance of the magnetic circuit and thus the electric current required to operate the motor.
The approach taught by German Patents DE 28 05 435 and DE 28 05 438 divides the stator into wound pieces and pole pieces. A non-magnetic material is inserted into the joints between the wound pieces and pole pieces, increasing the effective gap, and thus increasing the reluctance of the magnetic circuit and the electric current required to operate the motor. The layers of material that comprise the pole pieces are oriented with their planes perpendicular to the planes of the layers in the wound back iron pieces. This configuration further increases the reluctance of the stator, because contiguous layers of the wound pieces and of the pole pieces meet only at points, not along full line segments, at the joints between their respective faces. In addition, this approach teaches that the laminations in the wound pieces are attached to one another by welding. The use of heat intensive processes, such as welding, to attach amorphous metal laminations will recrystallize the amorphous metal at and around the joint. Even small sections of recrystallized amorphous metal will normally increase the magnetic losses in the stator to an unacceptable level.
Another difficulty associated with the use of ferromagnetic amorphous metals arises from the phenomenon of magnetostriction. Certain magnetic properties of any magnetostrictive material change in response to imposed mechanical stress. For example, the magnetic permeability of a component containing amorphous materials typically is reduced and the core losses increased when the component is subjected to stress. The degradation of soft magnetic properties of the amorphous metal device due to the magnetostriction phenomenon may be caused by stresses resulting from any combination of sources, including: (i) magnetic and mechanical forces during the operation of the electric motor; (ii) mechanical stresses resulting from mechanical clamping or otherwise fixing the bulk amorphous metal magnetic components in place; or (iii) internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material. As an amorphous metal magnetic stator is stressed, the efficiency at which it directs or focuses magnetic flux is reduced, resulting in higher magnetic losses, reduced efficiency, increased heat production, and reduced power. The extent of this degradation may be considerable depending upon the particular amorphous metal material and the actual intensity of the stresses, as indicated by U.S. Pat. No. 5,731,649. The degradation of core loss is often expressed as a destruction factor, i.e., a ratio of the core loss actually exhibited by a finished device and the inherent core loss of the constituent material tested under stress-free, laboratory conditions.
Moreover, amorphous metals have far lower anisotropy energies than other conventional soft magnetic materials, including common electrical steels. As a result, stress levels that would not have a deleterious effect on the magnetic properties of these conventional metals have a severe impact on magnetic properties important for motor components, e.g. permeability and core loss. For example, the ""649 patent further discloses that forming amorphous metal cores by rolling amorphous metal into a coil, with lamination using an epoxy, detrimentally restricts the thermal and magnetic saturation expansion of the coil of material, resulting in Is high internal stresses and magnetostriction that reduces the efficiency of a motor or generator incorporating such a core. In order to avoid stress-induced degradation of magnetic properties, the ""649 patent discloses a magnetic component comprising a plurality of stacked or coiled sections of amorphous metal carefully mounted or contained in a dielectric enclosure without the use of adhesive bonding.
A number of applications in current technology, including such widely diverse areas such as high-speed machine tools, aerospace motors and actuators, and spindle drive motors for magnetic and optical disk drives used for data storage in computers and other microelectronic devices, require electrical motors operable at high speeds, many times in excess of 15,000-20,000 rpm, and in some cases up to 100,000 rpm. The limitations of magnetic components made using existing materials entail substantial and undesirable design compromises. In many applications, the core losses of the electrical steels typically used in motor components are prohibitive. In such cases a designer may be forced to use a permalloy alloy as an alternative. However, the attendant reduction in saturation induction (e.g. 0.6-0.9 T or less for various permalloy alloys versus 1.8-2.0 T for ordinary electrical steels) necessitates an increase in the size of magnetic components comprised of permalloy or variants thereof. Furthermore, the desirable soft magnetic properties of the permalloys are adversely and irreversibly affected by plastic deformation, which can occur at relatively low stress levels. Such stresses may occur either during manufacture or operation of the permalloy component.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved amorphous metal motor components that exhibit a combination of excellent magnetic and physical properties needed for high speed, high efficiency electric machines. Construction methods are also sought that use amorphous metal efficiently and can be implemented for high volume production of motors of various types and of the components used therein.
The present invention provides a high efficiency electric motor comprising one or more low-loss bulk amorphous metal magnetic components having the shape of a polyhedron and being comprised of a plurality of layers of amorphous metal strips. Also provided by the present invention is a method for making a low core loss, bulk amorphous metal magnetic component. More specifically, a magnetic component constructed in accordance with one embodiment of the present invention and excited at an excitation frequency xe2x80x9cfxe2x80x9d to a peak induction level xe2x80x9cBmaxxe2x80x9d will have a core loss at room temperature less than xe2x80x9cLxe2x80x9d wherein L is given by the formula L=0.005 f (Bmax)1.50.000012 f1.5 (Bmax)1.6, the core loss, the excitation frequency and the peak induction level being measured in watts per kilogram, hertz, and teslas, respectively. Preferably, the magnetic component has (i) a core-loss of less than or approximately equal to 2.8 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 400 Hz and at a flux density of approximately 1.3 Tesla (T); (ii) a core-loss of less than or approximately equal to 5.7 watts-per-kilogram of amorphous metal material when operated at a frequency of approximately 800 Hz and at a flux density of approximately 1.3 T, or (iii) a core-loss of less than or approximately equal to 9.5 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 2,000 Hz and at a flux density of approximately 1.0 T.
As a result of its very low core losses under periodic magnetic excitation, the magnetic component of the invention is operable at frequencies ranging from DC to as much as 20,000 Hz or more. It exhibits improved performance characteristics when compared to conventional silicon-steel magnetic components operated over the same frequency range. The component""s operability at high frequency allows it to be used in fabricating motors that advantageously are operable at higher speeds and with higher efficiencies than are possible using components made with conventional materials.
The present invention also provides a method of constructing a bulk amorphous metal magnetic component. An implementation of the method includes the steps of forming a plurality of laminations of a predetermined requisite shape from ferromagnetic amorphous metal strip feedstock, stacking the laminations in registry to form a three-dimensional shape, and applying and activating adhesive means to adhesively bond the laminations to each other forming a lamination stack having sufficient structural and mechanical integrity. Advantageously, compressive and tensile stresses that result inherently from bending strip during winding are absent in a fabrication method that employs individually formed laminations. Any stress resulting from the formation of the laminations will likely be confined merely to a small region at or near the periphery thereof. Optionally the lamination stack is then finished to remove any excess adhesive and to give it a suitable surface finish and final component dimensions.
The formation of laminations in the requisite shape may be carried out in a number of ways, including non-exclusively cutting by mechanical grinding, diamond wire, high-speed milling performed in either horizontal or vertical orientation, abrasive water jet milling, electric discharge machining by wire or plunge, electrochemical grinding, electrochemical machining, stamping, laser cutting, or other means known to one having ordinary skill. Preferably, laminations are formed by a photolithographic etching technique. The adhesive bonding step may be carried out before or after the annealing step. The method may further comprise an optional heat treatment or annealing to improve the magnetic properties of the component or an optional coating step wherein an insulating coating is applied to at least a portion of the surface of the component. These steps may be carried out in a variety of orders and using a variety of techniques including those set forth hereinbelow. The preferred amorphous metal material preferably used in the practice of the method has a composition defined essentially by the formula Fe80B11Si9.
The present invention is also directed to a bulk amorphous metal motor component constructed in accordance with the above-described methods.
Bulk amorphous metal magnetic components constructed in accordance with the present invention are especially suited for use as amorphous metal stators or stator components in highly efficient, variable reluctance motors and eddy current motors. Similarly, bulk amorphous metal components may be used as both the rotor and the stator in squirrel cage motors, reluctance synchronous motors and switched reluctance motors. It will be understood by those skilled in the art that such motors may comprise one or more rotors and one or more stators. Accordingly, the terms xe2x80x9ca rotorxe2x80x9d and xe2x80x9ca statorxe2x80x9d as used herein with reference to motors mean a number of rotors and stators ranging from 1 to as many as three or more. It will further be recognized by those familiar with the rotating electrical machine art that radial flux motors may be constructed either: with (i) the rotor located within, and having a generally smaller diameter than, the stator or (ii) in the inside-out or cup configuration in which the relative positions and sizes of the rotor and stator are exchanged. A rotor or a stator of the invention may be constructed either as a unitary structure or as an assembly of a plurality of sub-structures held together by known means, the sub-structures being made as taught herein.
It will also be recognized by those skilled in the art that the term xe2x80x9celectric motor,xe2x80x9d as used herein, refers generically to a variety of rotating electrical machines which additionally comprise electric generators as well as regenerative motors that may be operated optionally as electric generators. The magnetic component of the invention may be employed in constructing any of these devices. Significant advantages are realized during use of the present invention. These advantages include simplified manufacturing and reduced manufacturing time, reduced stresses (i.e., magnetostrictive) encountered during construction of bulk amorphous metal components, optimized performance of the finished amorphous metal magnetic component, and improved efficiency of an electric motor comprising the rotor or stator disclosed herein.